Scanning electron microscopy - Principles and equipment

Scanning Electron Microscopy (SEM) is a powerful technique for observing samples and, in particular, surface topography. It is based primarily on the detection of secondary electrons emerging from the surface under the impact of a very fine beam of primary electrons that scans the surface of a sample and enables images to be obtained with a separating power often below 5 nm, and a great depth of field.

In addition to secondary electron emission, the SEM can also use other signals emitted by interactions between primary electrons and the sample: backscattered electrons, diffraction of backscattered electrons, absorbed electrons, electrons transmitted through a thin plate, as well as the emission of X-ray photons, sometimes photons close to the visible range, etc. These interactions are often indicative of the topography or composition of the surface, or the local crystalline orientation. These interactions are often indicative of surface topography or composition, or local crystalline orientation.

The instrument forms a very fine beam (down to the nanometer) of electrons accelerated by adjustable voltages from 0.01 to 30 kV, focuses it and scans it over the area of the sample to be examined. Appropriate detectors - specific electron detectors (secondary, backscattered, etc.), complemented by photon detectors, or others - are used to collect significant signals emitted as the surface is scanned, and to form various images then synchronized with the scan.

This article describes the design of the instrument, and the electron-matter interactions that drive imaging. The [P 866] article describes image formation, contrast sources, recent developments and applications.